1. Field of the Invention
This invention relates to the production of foamed articles.
2. Description of the Prior Art
To form a synthetic resin foam, it is necessary to disperse a foaming agent through a resin to be foamed, and this dispersion must be achieved before the foaming process or setting of the resin is too far advanced if a satisfactory foam of uniform and predictable qualities is to be produced.
Foams have been made using organic compounds and heat to produce gases for forming cells in the resin, but such methods have been relatively expensive or inconvenient. Polyester foams have also been made by first producing an emulsion of water and polyester resin and then adding blowing agents in the form of bicarbonates. The formation of an emulsion is necessary in order to enable rapid dispersion of the blowing agent through the resin before either the blowing reaction or the setting of the resin is too far advanced. Foams produced by this method must have an open cell structure, so they can be dried to remove the unwanted water that remains after foaming. Water in the emulsion greatly lowers the peak temperature attained during the exothermic cross-linking reaction and greatly reduces foam strength and its impact resistance. Such foams have not achieved widespread commercial success, and the principal use of polyester resins in structural work has been in glass fiber laminates made of successive layers each of which must be allowed to cure to substantial rigidity before another layer is applied. To obtain substantial stiffness and bending resistance it is necessary to build up a large number of such layers.
U.S. Pat. No. 2,461,942 issued to Walter Ten Brocek discloses a process for making soft sponge from vinyl chloride copolymers. In the process vinyl chloride copolymer together with a plasticizer and a gas evolving solid is heated in a mold to a temperature at which the vinyl chloride copolymer mixture is quite fluid and at which the blowing agent is unstable. This process produces a soft sponge material and due to the liberation of gas from the blowing agent in a mold at a relatively high temperature is expensive and requires highly specialized molding techniques which can only be preformed at a factory site.
U.S. Pat. No. 2,498,621 issued to Edward Kropa includes a process for making porous foam material from polymerizable composition by using blowing agents in the form of diazolized toluidine salt and heating the mixture and curing it at 140.degree. C., or adding a blowing agent in the form of ammonium bicarbonate and heating the mixture and curing it at 100.degree. C., or adding a blowing agent in the form of powdered Dry Ice and heating the mixture and curing it at 140.degree. C. to expand entrapped carbon dioxide or adding a blowing agent in the form of potassium carbonate and heating the mixture and curing it at 100.degree. C. All of these methods of producing foamed porous material from polymerizable composition using blowing agents which are liberated by application of heat which in the production of foamed objects makes the manufacturing process more expensive and confined to the factory site.
U.S. Pat. No. 3,260,683 issued to Watanabe et al discloses a process making porous material from unsaturated polyester resin by using any blowing agent and anionic surfactants which consist of salts of organic ester of phosphoric acid or organic sulfonic acid salts. A catalyst and an accelerator are used for curing the foamed unsaturated polyester resin composition. The process is relatively expensive and results in a very light density foam suitable for heat insulation but brittle and not suitable for the manufacture of products requiring high strength. Also anionic surfactants which consist of salts of organic esters of phosphoric acid or organic sulfonic acid salts become ineffective when used with strong inorganic acids as components of blowing agent.
U.S. Pat. No. 3,479,303 issued to Helmut Wieschollek discloses a process for making porous foam material from unsaturated polyester resin using as blowing agents carbonates of the alkali metals and alkaline earth metals such as sodium carbonate, sodium bicarbonate, ammonium bicarbonate and calcium bicarbonate which react with organic acids like lower apiphatic acids, formic acid, acetic acid etc. to liberate carbon dioxide. Since such acids can only dissociate sufficiently in the presence of adequate water an emulsion of water and unsaturated polyester resin is prepared and a blowing agent, organic acid, hardener and accelerator added to obtain a porous structure which is of an open cell type and contains large amounts of water. In a product composed of such a foam, covered with a skin, like for example, polyester saturated fiber glass mat or cloth this water cannot be disposed of by drying and it produces a detremental effect as far as the physical characteristics of the foam are concerned.